Electrostatics and Rheology of Unentangled Semidilute Polyelectrolyte Solutions

Polyelectrolytes (PE) are charged polymers in polar solvents. Classical scaling theory suggests that the viscosity η for salt-free semidilute unentangled PE solutions in Θ solvents obeys the empirical Fuoss law η∼c^1/2 in the higher polymer concentration (c) regime, and η∼c^5/4 in the lower c regime. However, recent experiments have also reported η∼c^0.68 and η∼c^0.91, which are at odds with the classical scaling theory. To rationalize the four distinct scaling laws, we probe the electrostatic energy per monomer under the influence of salt and their contributions to the viscosity of PE solutions. We identify four consecutive regimes dependent on the magnitude of the ratio of the polymer concentration over the salt concentration, which capture the unexplained observations and classical PE solution theories, and provide physical insights for the effects of salt contamination and added salt on the properties of both weakly and strongly charged semidilute unentangled PE solutions.